1. Field of the Invention
The present invention relates to a controller for a cutting tool drive motor of a dental handpiece which is used to cut teeth and to form root canals by driving a cutting tool installed at the tip of the handpiece.
2. Description of the Prior Art
Since a motor for dental treatment is mainly used to drive a cutting tool for cutting teeth or the like, a drive method has been adopted, wherein the rotation speed of the motor is not reduced but maintained at a preset speed even when the motor undergoes a large load. As a general example of such a method, feedback control has been known, wherein the rotation speed of the motor detected by a sensor installed in a handpiece is compared with a preset speed, and the voltage for driving the motor is controlled so as to eliminate the difference between the two speeds, thereby maintaining the rotation speed constant. The output of the motor is adjusted by changing the preset speed.
However, as the cutting tool becomes slender, it is apt to break easily, since the rotation speed is maintained constant regardless of the load applied to the cutting tool. Therefore, a relatively slender cutting tool, for example a file for forming root canals, more particularly a Ni-Ti file specified to have the maximum rotation speed of 300 rpm, is apt to break very easily, thereby causing the problem of making dental operation difficult.